Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (unit cell). The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, a predetermined numbers of the unit cells and the separators are stacked together to form a fuel cell stack.
In the fuel cell, a gas chiefly containing oxygen or the air (hereinafter also referred to as the “oxygen-containing gas”) is supplied to the cathode. The oxygen in the oxygen-containing gas is ionized at the interface between the cathode and the electrolyte, and the oxygen ions (O2−) move toward the anode through the electrolyte. A fuel gas such as a gas chiefly containing hydrogen (hereinafter also referred to as the “hydrogen-containing gas”) or CO is supplied to the anode. The oxygen ions react with the hydrogen in the hydrogen-containing gas to produce water or react with CO to produce CO2. Electrons released in the reaction flow through an external circuit to the cathode, creating a DC electric energy.
For example, in a solid oxide fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2002-203579, as shown in FIG. 23, power generation cells 1 and separators 2 are stacked alternately. Each of the power generation cells 1 includes an electrolyte layer 1a of solid electrolyte, and a fuel electrode layer 1b and an air electrode layer 1c provided on both surfaces of the electrolyte layer 1a. An electrically conductive porous fuel electrode current collector 3 is interposed between the power generation cell 1 and one of separators 2 sandwiching the power generation cell 1, and an electrically porous air electrode current collector 4 is interposed between the power generation cell 1 and the other of the separators 2.
A fuel supply passage 5 and an air supply passage 6 are formed on the separators 2. The fuel supply passage 5 and the air supply passage 6 are provided at substantially the center of the separator 2. The fuel supply passage 5 is connected to a fuel hole 5a facing the fuel electrode current collector 3 on one surface of the separator 2, and the air supply passage 6 is connected to an air hole 6a facing the air electrode current collector 4 on the other surface of the separator 2.
In the structure, the fuel gas (H2, CO or the like) flows through the fuel supply passage 5, and is discharged from substantially the central region of the separator 2 to the central region of the fuel electrode current collector 3. Therefore, the fuel gas flows through apertures in the fuel electrode current collector 3, and is supplied to the substantially central region of the fuel electrode layer 1b. Further, the fuel gas is guided by slits (not shown), and flows radially from the substantially central region to the outer region of the fuel electrode layer 1b. 
In the meanwhile, the air flows through the air supply passage 6, and is discharged from the substantially central region of the separator 2 to the central region of the air electrode current collector 4. Therefore, the air flows through apertures in the air electrode current collector 4, and is supplied to the substantially central region of the air electrode layer 1c. Further, the air is guided by slits (not shown), and flows radially from the substantially central region to the outer region of the air electrode layer 1c. Thus, power generation is performed in each of the power generation cells 1.
In the conventional technique as described above, the fuel gas flows from the substantially central region to the outer region of the fuel electrode layer 1b, and the air flows from the substantially central region to the outer region of the air electrode layer 1c. Therefore, the unconsumed fuel gas and air are mixed together, and combusted around the outer region of the power generation cell 1. After combustion, the mixed gas is discharged to the outside as an exhaust gas. At this time, since the flow rate of the supplied air is larger than the flow rate of the supplied fuel gas, oxygen remains in the exhaust gas. The outer circumferential region of the power generation cell 1 is likely to be exposed to the oxygen remaining in the exhaust gas.
The fuel electrode layer 1b is made of metal such as nickel (Ni). The outer circumferential region (Ni) of the fuel electrode layer 1b is oxidized to NiO. The exhaust gas containing oxygen moves into the fuel electrode current collector 3. Thus, reduction reaction of Nio of the fuel electrode layer 1b is prevented. Since NiO has high electrical resistance, it reduces the effective surface area of the anode used for power generation. Thus, the overall power generation performance (efficiency) of the power generation cell 1 is lowered.